Polypropylene (PP) exhibits higher melting point and lower density. It shows excellent chemical resistance, higher tensile modulus and is obtained at lower cost. This is the reason that it has already captured a major market share of commodity plastics. However commercial PP is constituted of highly linear chains with a relatively narrow molecular weight distribution. It shows poor processing characteristics in processes where extensional stiffing is predominantly required. In order for PP to be used by shaping processes like foaming, thermoforming, extrusion coating, blow molding etc, modifications are needed to enhance the strain hardening behavior (manifestation of high melt strength) of its melt. Even though a very broad (including bimodal) molecular weight distribution (MWD) can improve this behavior, strain hardening is most efficiently achieved by the addition of long chain branching (LCB). It is expected that if the melt strength behavior of PP is improved, its market position will become even more prominent, replacing thermoplastics in several applications.
Several commercial grades of (HMS-PP) are available, mostly developed using electron beam (EB) irradiation [M. Ratzsch, Pure Appl. Chem., A36, 1759 (1999)]. The EB irradiation causes scission of the PP chains, which is followed by some recombination reaction, leading to long chain branching as disclosed in A. J. DeNicola, A. F. Galambos, and M. D. Wolkowicz, “Radiation Treatment of Polypropylene.” Polymeric Materials Science and Engineering, Volume 67, 106 (Fall Meeting, 1992); B. J. Scheve, J. W. Mayfield, and A. J. DeNicola, U.S. Pat. No. 4,916,198, Himont Inc. (1990) & A. J. DeNicola, European Patent No. 0384431B1, Himont Inc. (1989).
Basell's patents (WO:2010:076701, U.S. Pat. No. 4,916,198) claim the production of long chain branching on PP by irradiating with an electron beam under oxygen free atmosphere followed by two heating steps to allow radical recombination and annihilation reaction. Some others companies have issued patents using electron beam processing, but so far there is not actual production other than Basell one. As a result of a research joint effort, IPEN, BRASKEM (the biggest Brazilian polymer producer) and EMBRARAD (the major Brazilian radiation processing center) developed a new process to produce HMS PP based on gamma processing.
Modified polypropylene commercially available as Profax [R] by Basell and Daploy[R] by Borealis, has been used successfully in foaming, thermoforming and extrusion coating processes.
Low decomposition temperature peroxides are also used to induce long chain branching. U.S. Pat. No. 5,047,485, Himont Inc. (1991) U.S. Pat. No. 5,416,169, Chisso (1994) disclose the use of peroxydicarbonates to modify PP at low temperatures.
WO Patent 97/49759 and WO Patent 99/27007 patents disclose reactive extrusion of PP with several peroxydicarbonates (PODIC).
The modifications in all these developments result in the grafting of long chain branches on the PP backbone. Even though most commercial HMS-PP is produced now by electron beam irradiation, the method of using peroxydicarbonates and reactive extrusion has recently regained interest, as it can be directly applied also by the foam manufacturer on commercial linear PP. It is well known that the melt strength of polyolefin increases with LCB. Ghijssels studied the relation between the melt strength (MS) and the melt flow index (MFI) for polyethylene grades with different degrees of LCB and found their melt strength increase with decreasing MFI. However, the MS of LDPE was found to be higher than the one of LLDPE and HDPE for the same MFI by a factor of at least two as referred in V. V. De Maio and D. Dong, “The Effect of Chain Structure on Melt Strength of Polypropylene and Polyethylene.” SPE ANTEC Tech. Papers, 43, 1512 (1997) & A. D. Gotsis and Qinfei Ke, “Comparison of Three Methods to Measure the Elongational Viscosity of Polymer Melts: Entry Flow, Fiber Spinning and Uniaxial Elongation.” SPE ANTEC Tech. Papers, 1156 (1999).
The higher melt strength of LDPE is due to long chain branches, which introduces strain hardening on stretching. This effect is stronger with the “tree-type” than with the “comb-type” long chain branching. No differences in strain hardening were seen by using different co-monomers (1-butene, 1-hexene and 1-octene) in LLDPE. It seems that all these side groups are too short to influence the elongational flow properties of the polymer melt & thus, the melt strength. In the case of PP, the melt strength also increases strongly with decreasing MFI as well as by widening the molecular weight distribution. Besides, branched polypropylenes obtained using electron beam irradiation were found to have ten times higher melt strength than a linear PP with the same MFI.
The elongational flow behavior of initially linear PP that is modified by reactive extrusion to obtain a long-chain-branched structure is affected by the molecular weight and molecular weight distribution of the precursor polymer. Broader precursor molecular weight distribution results in better thermoforming processing properties of the branched product. An optimum balance is found in these properties at a certain degree of branching, which also depends on the molecular weight and the molecular weight distribution of the polymer.
The elasticity of the polymer in all its manifestations is enhanced by long chain branching. Strain hardening index, an index defined here to characterize the degree of strain hardening of the melt, increases with the increase of the number of branches per molecule. The melt strength is enhanced by the addition of branches.
Some degree of long chain branching is beneficial for the foaming process. However, a very large number of branches per chain may reduce the foam ability of PP because they may reduce the strain at break of the melt. Peroxide-induced cross linking should also be avoided by the judicious choice of the peroxide and its amount used for the modification.
Conventional propylene polymeric materials have long been used in processes like thermoforming, blow molding, coating, etc requiring high melt strength which could be achieved by increasing molecular weight and broadening of molecular weight distribution. Molecular weight and molecular weight distribution can be modified in the polymerization process itself by choosing particular process conditions and catalyst type. However, typical propylene polymer resins, even those having high molecular weight and broad molecular weight distribution often cannot provide commercially desired levels of melt strength without additional processing. Techniques to improve melt strength have included irradiation of conventional flake polypropylene in reduced-oxygen environments, as described, in U.S. Pat. Nos. 4,916,198, 5,047,485, 5,414,027, 5,541,236, 5,554,668, 5,591,785, 5,731,362, and 5,804,304.
U.S. Pat. No. 5,047,485, discloses a process for producing a propylene polymer with free-end long chain branching by mixing a low-decomposition-temperature peroxide with a linear propylene polymer in the substantial absence of atmospheric oxygen, heating the resulting mixture to 120° C., and then deactivating substantially all the free radicals present in the propylene polymer. The processing temperature must be sufficient to decompose the low decomposition temperature peroxide but low enough to favor the recombination of the polymer fragments. It is further taught that processing temperatures above 120° C. provide a product with little or no branching (i.e. an essentially linear polymer).
U.S. Pat. No. 5,541,236 discloses a solid-state process for making a high melt strength propylene polymer by the formation of free-end long branches through irradiating linear propylene polymer material in a substantially oxygen-free environment (less than about 15% oxygen by volume) with high energy radiation to produce a substantial amount of molecular chain scission, maintaining the irradiated propylene polymer in the substantially oxygen-free environment to allow chain branches to form, and then deactivating substantially all the free radicals present in the irradiated propylene polymer material.
In the presence of free radicals formed from irradiation or peroxide reaction at higher temperatures, branching and chain scission (i.e. fragmentation) of polypropylene occur simultaneously, with chain scission mechanisms dominating due to first order kinetics. In contrast, the effect of free radicals in the presence of polyethylene leads to crosslinking by macro radical recombination (i.e., covalent bonds may be formed that link the crystalline and amorphous regions of polyethylene into a three-dimensional network).
A peroxide-initiated degradation of polypropylene may be used for production of controlled rheology resins with tailor-made properties, narrowed molecular weight distribution, lowered weight average molecular weight, and increased melt flow rate, as described, for example, in U.S. Pat. No. 4,451,589. The degradation or breaking of polypropylene chains as described therein results in an undesirable lowering of melt strength for the polymer (i.e., chain scission results in lower molecular weight and higher melt flow rate polypropylenes than would be observed were the branching not accompanied by scission).
The irradiation methods increase propylene polymer melt strength by creating polymer radicals during irradiation which then recombine to form long-chain branches in the controlled oxygen environment. Irradiation of syndiotactic and atactic metallocene-derived polymers has been described in U.S. Pat. Nos. 5,200,439 and 6,306,970, respectively. Irradiation of material having a Mw/Mn less than 2 generated by fragmentation of conventional polypropylene has been described in the Journal of Applied Polymer Science, Vol. 11, pp 705-718 (1967).
Other techniques for improving melt strength include irradiation of propylene polymer material in air, as described in U.S. Pat. No. 5,439,949. However, the increased oxygen levels favor chain scission reactions at the expense of branching reactions, which requires irradiation doses at or above the gelation point, thereby risking product quality and homogeneity.
Irradiating pellets of polymer material in air, as described in U.S. Patent Publication Number 2006/0167128, has been attempted to limit oxygen exposure, however, melt strength may still be adversely affected by chain scission occurring at the outer surface of the pellets.
Phenolic antioxidants have long been used to improve polymer stability under elevated temperature conditions, such as those typically experienced during extrusion, or during extended periods of storage. However, their use in irradiated compositions undermines enhanced melt strength by scavenging free radicals, thereby reducing the number of polymeric free radicals available to recombine to form long-chain branches. Moreover, irradiation of phenolic antioxidant-containing polymers can result in the formation of degradation products that impart undesirable color. Non-phenolic stabilizers have been used in the irradiation of conventional polyolefin materials to avoid such problems, as described in U.S. Pat. No. 6,664,317 and U.S. Provisional Patent Application No. 60/937,649, which has now published as International Application Publication No. WO2009/003930 and U.S. Patent Application Publication No. 2010/0113637, which has now issued as U.S. Pat. No. 8,399,536.
A significant challenge associated with production of high melt strength propylene materials via irradiation is the low melt flow rates typically required in the starting material to be irradiated. Low melt flow material (high viscosity) is normally used to ensure that the viscosity after irradiation is still sufficient for the needs of the application, as well as to provide long-chain radicals to help in melt strength development. However, such low melt flow rate material is also more difficult to process in plant equipment, and can result in production loss.
U.S. Pat. No. 3,970,722 discloses a method for preparing a modified polypropylene as a bonding agent by mixing crystalline propylene polymer, 0.1 to 5% organic peroxide with a half-life of one minute, and 0.1 to 7% modifying agent. The modifying agent may be either: (1) acrylic and methacrylic salts of Na, Ca, Mg, Zn, Al and Fe (III) or (2) compounds containing a phenol or benzyl group (e.g., 4-methacryloyl-oxymethylphenol). Because an excessive amount of organic peroxide may result in an increased melt flow index for the modified propylene polymer, it is taught that a non-modified crystalline propylene polymer in an amount of 50% or less may be added to the modified mixture in order to reduce the melt flow index to 120 or less. Also disclosed is that the organic peroxide should decompose completely during the preparation of the modified propylene polymer to prevent the decomposition of the non-modified crystalline propylene polymer added after modification.
An alternative method for introducing functional groups onto the polymer is described in U.S. Pat. No. 5,447,985. This process involves the addition of a peroxide (e.g. t-butyl peroxy maleic acid) having an activated unsaturation within the peroxide molecule and the optional addition of a co-agent (e.g., triallyl cyanurate, triallyl isocyanurate, ethylene glycol dimethacrylate, and trimethylolpropane trimethacrylate). The patent teaches that the activating group in the peroxide is a carboxylic acid group and that the melt flow index of the (co)polymer is significantly increased by the peroxide modification.
Grafting low molecular weight side chains onto peroxygenated polyolefins is known in the prior art. U.S. Pat. No. 6,444,722 discloses a process for making graft copolymers by treating the peroxygenated polyolefin in a substantially non-oxidizing atmosphere at a temperature of about 110° to 140° C. with at least one grafting monomer in liquid form and at least one additive to control the molecular weight of the side chains. It is disclosed that there is a need to control the molecular weight of the polymerized monomer side chains of polypropylene graft copolymers made from the per oxygenated polyolefin so that low molecular weight side chains are produced without adversely affecting the overall physical properties of the graft copolymer.
In U.S. Pat. No. 6,774,186, the free radical co-agent is a monomer or low molecular weight polymer having two or more functional groups with high response to free radicals. Typically, these functional groups are methacrylate, allyl or vinyl types. The free radical from peroxides enhances the rheological modification. Firstly, by peroxide induced allylic hydrogen abstraction from the co-agent, a lower energy state, longer-lived free radical is created. This free radical can then induce branching in the ethylene elastomer by hydrogen abstraction. Due to the lower energy state of the free radical, beta-scission and disproportionation of either polypropylene or ethylene elastomeric phase is less likely to occur. Secondly, the multifunctional co-agent can act as a bridging group between the polymer chains. Suitable co-agents for this application would include diallyl terephthalate, triallylcyanurate, triallylisocyanurate, 1,2 polybutadiene, divinyl benzene, trimethylolpropane trimethacrylate, polyethylene glycol dimethacrylate, ethylene glycol dimethacrylate, pentaerythritol triacrylate (PETA), allyl methacrylate, N N′-m-phenylene bismaleimide, toluene bismaleimide-p-quinone dioxime, nitrobenzene, diphenylguanidine. Preferred co-agents are triallylcyanurate, 1,2 polybutadiene, divinyl benzene, and trimethylolpropane trimethacrylate (TMPTA). The co-agent is suitably present in an amount that is within the range of about 100 to 10,000 parts per million by weight. The peroxide and co-agent can be added by any conventional means. Illustrative procedures include imbibing it onto polymer pellets prior to compounding, adding it to polymer pellets as the pellets enter a compounding apparatus such as at the throat of an extruder, adding it to a polymer melt in a compounding apparatus such as a Haake, a Banbury mixer, a Farrel continuous mixer or a Buss-co-kneader or injecting it into an extruder, at 100% active ingredients (i.e., neat) or optionally as a dispersion or solution in an oil, such as a processing oil, at a point where the extruder contents are in molten form.
Grafting short chain branches or functional groups onto semi crystalline polypropylene resins, however, has proven to be insufficient to enhance the melt strength of such resins. Poor melt strength of polypropylenes can be seen in properties such as, e.g., excess sag in sheet extrusion, rapid thinning of walls in parts thermoformed in the melt phase, low draw-down ratios in extrusion coating, poor bubble formation in extrusion foam materials, and relative weakness in large-part blow molding. In addition, the use of free radical generators, such as organic peroxides, having a highly concentrated peroxide content (i.e., greater than 400 mmoles/kg) must be carefully controlled in order to keep the degradation (e.g., increased melt flow rate) of the polypropylene resin to a minimum. Accurately metering such low levels of peroxide in grafted propylene production is very difficult even when an organic peroxide master batch with low peroxide content is used.
Therefore, to obviate the disadvantages associated with the prior art, a need is felt to produce melt strength propylene polymers using simple and cost effective process.